Topic C4 - Chemical Changes Flashcards
Revise topic C4
What are the lowest and highest pHs?
0 and 14
What colour does an acid turn the universal indicator?
What colour does an alkaline turn the universal indicator?
Acid = red alkalis = purple
Is an acidic pH low or high?
Is an alkaline pH low or High?
Acidic = low alkali = high
What ions do acids form in water?
H+
What ion do alkalis form in water?
OH-
What is a base?
A substance with a pH larger than 7
What is pH measuring?
The concentration of hydrogen ions
For every decrease of 1 on the pH scale, does the concentration of H+ ions increase or decrease, and by how much?
What about for a decrease of 2?
When the pH decreases by 1 the concentration of H+ ions increases by a factor of 10
when the pH decreases by 2 the concentration of H+ ions increases by a factor of 100
What is the reactivity series?
Potassium Sodium Lithium Calcium Magnesium Carbon Zinc Iron Hydrogen Copper
How can you tell the reactivity of a metal by reacting it with an acid?
The more reactive the metal the more bubbles will be given off at a faster rate. you can also tell by how much the temperature changed, and how long the reaction lasted (the faster it goes the more reactive the acid)
What do you get when you react an acid and a metal?
Salt and hydrogen
What do you get when you react a metal and water?
Metal hydroxide + hydrogen
What is the definition of an ore?
A metal oxide that is naturally found in rocks and contains enough metal to be economical to extract
What is the opposite of oxidation?
Reduction
Why can’t some metals be extracted using carbon?
They are more reactive than carbon, so carbon can’t displace them.
Oxidation and reduction are both examples of what type of reaction?
A redox reaction
What what is the name of the reaction that means ‘splitting up with electricity’?
Electrolysis
If electrons are transferred, what type of reaction is it?
A redox reaction
Define a strong acid.
Define a weak acid.
A strong acid forms H+ ions and ionises fully in aqueous solutions.
A weak acid forms H+ ions but only partially ionises in aqueous solutions.
Define a concentrated acid.
Define a dilute acid.
A concentrated acid has a large mass of acid per unit volume. A dilute acid has a small mass of acid per unit volume.
Are metal oxides and hydroxides acidic or alkaline?
Alkaline
What does a reaction between an acid and a metal carbonate produce?
A salt, water and carbon dioxide
What does a reaction between an acid and a metal oxide produce?
What about with a metal hydroxide?
They both produce a salt and water.
What does a reaction between a metal and an acid produce?
A salt and hydrogen
what is an indicator?
a dye that changes colour depending on whether it’s above or below a certain pH.
what are wide range indicators?
indicators that contain a mixture of dyes that means they gradually change colour over a broad range of pH
what are wide range indicators useful for?
estimating the pH of a solution
which is more accurate, universal indicator or a pH probe + meter?
a pH probe + meter
how can you measure pH electronically?
- attach a pH probe to a pH meter
- place the probe in the solution you are measuring and the pH is given on a digital display as a numerical value, meaning it’s more accurate than an indicator
what is an acid?
a substance that forms aqueous solutions with a pH of less than 7. Acids form H+ ions in water
what is an alkali?
a base that dissolves in water to form a solution with a pH greater than 7. Alkalis form OH- ions in water
what is neutralisation?
the reaction between acids and bases
give the formula for the reaction between an acid and a base
acid + base -> salt + water
write the formula for neutralisation between acids and alkalis in terms of H+ and OH- ions
H+ + OH- -> H2O
how can an indicator be used to show that a neutralisation reaction is over?
the products of a neutralisation reaction are neutral (i.e. they have a pH of 7), therefore an indicator can be used to show that a neutralisation reaction is over
what do acid particles do in water?
they dissociate to release H+ ions
what type of reaction is the ionisation of a weak acid? What does this allow to happen?
the ionisation of a weak acid is a reversible reaction, which sets up an equilibrium between the undissociated and dissociated acid.
where does the position of equilibrium lie in the ionisation of a weak acid? why?
since only a few of the acid particles release H+ ions, the position of equilibrium lies well to the left.
will strong acids be more or less reactive than weak acids of the same concentration? why?
more reactive - reactions of acids involve the H+ ions reacting with other substances. If the concentration of H+ ions is higher, the rate of reaction will be faster, so strong acids will be more reactive than weak acids of the same concentration
what is the formula for the change in H+ ion concentration if you take the difference in pH to be x?
10^-x
(x is the difference in pH. So if pH falls from 7 to for the difference is -3, and the factor the H+ ion concentration has increased by is 10^-(-3) = 10^3 = 1000)
name 3 strong acids
- sulfuric acid
- nitric acid
- hydrochloric acid
why do all metal oxides and metal hydroxides react with acids to form a salt and water?
metal oxides and metal hydroxides are bases, and acids and bases react together to form a salt and water
are metal carbonates acidic or alkaline?
alkaline
what are the 5 steps to making soluble salts using an insoluble base?
- you need to pick the right acid and insoluble base, such as an insoluble metal oxide, hydroxide, or carbonate. E.g. if you want to make copper chloride, you could mix hydrochloric acid and copper oxide
- gently warm the dilute acid using a bunsen burner, then turn off the bunsen burner
- add the insoluble base to the acid a bit at a time, until no more reacts (i.e. the base is in excess). You’ll know when all the acid has been neutralised because, even after stirring, the excess solid will just sink to the bottom of the flask
- then filter out the excess solid to get to the salt solution
- to get pure, solid crystals of the salt, gently heat the solution using a water bath or an electric heater to evaporate some of the water (to make it more concentrated) and then stop heating it and leave the solution to cool. Crystals of the salt should form, which can be filtered out of the solution and then dried. This is called crystallisation
what is the reactivity of metals determined by?
how easily they lose electrons - forming positive ions. The higher up the reactivity series a metal is, the more easily they form positive ions.
give a method of placing metals in order of reactivity
when metals react with water or acid, they lose electrons and form positive ions. So, the higher a metal is in the reactivity series, the more easily it reacts with water or acid. If you compare the relative reactivity of different metals with either an acid or water and put them in order from most reactive to the least reactive, the order you get is the reactivity series
describe a method for finding out how reactive a metal is
some metals react with acids to produce a salt and hydrogen gas:
- the speed of reaction is indicated by the rate at which the bubbles of hydrogen are given off
- the more reactive the metal, the faster the reaction will go. Very reactive metals like potassium, sodium, lithium and calcium react explosively, but less reactive metals such as magnesium, zinc and iron react less violently. In general, copper won’t react with cold, dilute acids
- you can also investigate the reactivity of metals by measuring the temperature change of the reaction with an acid or water over a set time period. If you use the same mass and surface area of metal each time, then the more reactive the metal, the greater the temperature change should be
name 4 metals that will react with water
potassium, sodium, lithium and calcium will all react with water
name 3 metals that won’t react with water
- zinc
- iron
- copper
give the balanced symbol equation (with state symbols) for the reaction between calcium and water
Ca(s) + 2H2O -> Ca(OH)2(aq) + H2(g)
give two ways to describe oxidation
- the addition of oxygen
2. a loss of electrons
give 2 ways to describe reduction
- the removal of oxygen
2. the gaining of electrons
are iron atoms oxidised or reduced when they react with dilute acid?
they are oxidised by the hydrogen ions (they lose electrons)
when iron reacts with a dilute acid, are the hydrogen ions in the acid oxidised or reduced?
they are reduced by the iron atoms - they gain electrons
what type of reactions are displacement reactions?
redox reactions
choose the right words:
in a displacement reaction, the more/less reactive substance/metal/non-metal will displace the more/less reactive substance/metal/non-metal from its compound.
in a displacement reaction, the more reactive metal will displace a less reactive metal from its compound
in a displacement reaction between iron and copper (iron replacing copper), which metal is oxidised and which is reduced?
- the iron loses 2 electrons to become a 2+ iron - it’s oxidised
- the copper gains these 2 electrons to become a copper atom - it’s reduced
in displacement reactions, is it the metal ion or the metal atom that gains electrons and is reduced?
the metal ion
in displacement reactions, is it the metal ion or the metal atom that loses electrons and is oxidised?
the metal atom
what do ionic equations show?
only the particles that react and the products they form are shown - it just shows the useful bits of a reaction are shown. For example, the full equation for the displacement of zinc ions by magnesium metal is: Mg(s) + ZnCl2(aq) -> MgCl2(aq) + Zn(s), whereas the ionic equation would be Mg(s) + Zn2+ -> Mg2+ + Zn(s)
write an ionic equation for the reaction of zinc and iron sulfate (Zn + FeSO4 -> ZnSO4 + Fe)
Zn + Fe[2+] -> Zn[2+] + Fe
in the reaction between zinc and iron sulfate, which species is being reduced and which is being oxidised?
iron is being reduced and zinc is being oxidised
what happens during electrolysis?
an electric current is passed through an electrolyte (a molten or dissolved isotonic compound). The ions move towards the electrodes, where they react, and the compound decomposes. The positive ions move towards the cathode and the negative electrons move towards the anode. This creates a flow of charge through the electrolyte as ions travel to the electrodes. As ions gain or lose electrons, they form an uncharged element and are discharged from the electrolyte
during electrolysis, where do the positive ions in the electrolyte move? do they lose or gain electrons? are they oxidised or reduced?
the positive ions in the electrolyte will move towards the cathode and gain electrons (they are reduced)
during electrolysis, where do the negative ions in the electrolyte move? do they lose or gain electrons? are they oxidised or reduced?
the negative ions in the electrolyte will move towards the anode and lose electrons (they are oxidised)
what is an electrolyte? what is an electrode?
a liquid or solution that can conduct electricity. An electrode is a solid that conducts electricity and is submerged in the electrolyte
why can a molten ionic compound be electrolysed, but not an ionic solid?
Molten ionic compounds can be electrolysed because the ions can move freely and conduct electricity. An ionic solid can’t be electrolysed because the ions are in fixed positions and can’t move.
why should the electrodes be inert during electrolysis?
so they they don’t react with the electrolyte
why is using electrolysis to extract metals very expensive?
lots of energy is required to melt the ore and produce the required current
what are the steps to extracting aluminium using electrolysis?
- aluminium is extracted from the ore bauxite by electrolysis. Bauxite contains aluminium oxide, Al2O3
- aluminium oxide has a very high melting temperature so it’s mixed with cryolite to lower the melting point
- the molten mixture contains free ions - so it’ll conduct electricity
- the positive Al[3+] ions are attracted to the negative electrode where they each pick up three electrons and turn into neutral aluminium atoms. These then sink to the bottom of the electrolysis tank.
- the negative O[2-] ions are attracted to the positive electrode where they each lose two electrons. The neutral oxygen atoms will then combine to form O2 molecules
why does the anode need to be replaced regularly when extracting aluminium from aluminium oxide?
the anode is made of carbon and needs to be replaced regularly as it reacts with oxygen to produce carbon dioxide
what are the rules for the cathode during electrolysis of aqueous solutions?
hydrogen will always be produced unless there is a metal less reactive than hydrogen present
what is the reactivity series?
potassium sodium lithium calcium magnesium carbon zinc iron hydrogen copper
what are the rules at the anode during electrolysis of aqueous solutions?
if OH- and halide ions (Cl-, Br-, I-) are present, molecules of chlorine, bromine or iodine will be formed. If no halide ions are present, then the OH- ions are discharged and oxygen will be formed (oxygen will always be formed unless there is a halogen present)
what do you need to remember to include when drawing the apparatus for an electrolysis experiment?
a d.c. power supply, wires and labels for the anode and cathode. The anode is the electrode on the same side as the longer line of the d.c. power supply symbol.
what do half equations show?
the reactions at the electrodes during electrolysis
what is the important thing to remember when you’re combining half equations?
the number of electrons needs to be the same for each half equation
give the half equations for the reactions at the anode and cathode during the electrolysis of aqueous sodium chloride. What do these equations combine to form?
anode: 2Cl[-] -> Cl2 + 2e[-] OR 2Cl[-] - 2e[-] -> Cl2
cathode: 2H{+} + 2e{-} -> H2
these combine to form the ionic equation: 2H{+} + 2Cl{-} -> H2 + Cl2
what is the half equation for the anode when a halide isn’t present in the aqueous solution?
4OH[-] -> O2 + 2H2O +4e[-] OR 4OH[-] - 4e[-] -> O2 + 2H2O
